Serological studies indicated that an agent closely related to human T- cell lymphotropic virus type I (HTLC-I) was occurring among the chimpanzees held by the Laboratory of Central Nervous System Studies. Existing assays could not distinguish whether the antibodies were against human or simian agents. Evidence of exposure to this virus or viruses was detected in 31 of 163 (19%) chimpanzees. In order to determine possible origins of the virus, DNA was extracted from lymphocytes of seropositive animals and 253 to 763 base pair sequences of the env, pol and LTR regions amplified by nested polymerase chain reaction (PCR). Proviral DNA was successfully amplified from 29 of 31 animals. DNA was sequenced directly from 10 animals as pools of 10 separate PCR amplifications to minimize Taw polymerase artifacts using commercial sequencing kits. Characterization and phylogenetic analysis of the sequences revealed that all of the isolates were strains of simian T-cell lymphotropic virus type I (STLV-I) of either of two classes, one naturally associated with West African chimpanzees and the other similar to known East African chimpanzees and Cercopithecus STLV- I.